A typical generator arrangement will include a shaft which transmits torque from a turbine to rotate a rotor in the generator to generate electricity. The rotor shaft also extends from the other side of the generator (often referred to as the exciter side) for support by a bearing. Generally, within the generator, an environment is provided which includes a gas such as hydrogen or helium for better heat conduction and reduced windage losses. On the shaft side which transmits the torque (turbine side) as well as the shaft side which provides support (exciter side), seals are provided for preventing escape of the gas, and also for preventing ingress of moisture or air to the interior of the generator. The seals may be in the form of a gland seal which directs two flows of oil toward and along the shaft. One flow of oil (hereinafter the hydrogen-side seal oil) will flow along the shaft toward the generator to prevent the escape of hydrogen to the outside atmosphere. Another flow (air-side seal oil) flows outwardly toward the bearing to prevent access of air or moisture into the generator. Since the oils tend to absorb hydrogen, air and moisture, the oil feed systems are separate, thereby preventing release of hydrogen to the outside atmosphere, and maintaining a high hydrogen purity within the generator. For best operation, the temperatures of the air-side seal oil and hydrogen-side seal oil should be maintained at or near the same temperature, and should be kept within 4.degree. F. If the oils are not at or near the same temperature, the seal ring can become distorted or non-uniformly thermally altered, thereby producing vibrations as the shaft rotates. Ring distortions can produce a rubbing between the shaft and the ring of the seal, which can in turn generate localized heating of the shaft causing it to bend and producing vibrations. Thus, it is essential to maintain the air-side seal oil and hydrogen-side seal oil at or near the same temperature.
Currently, two methods are available for temperature control of the hydrogen- and air-side oils. In one method, separate manual controls of cooling water to a cooler for the hydrogen-side oil, and a cooler for the air-side oil are utilized to control the respective oil temperatures. However, manual control requires constant adjustment to maintain the same or approximately the same temperatures of the two oils. Constant adjustment is necessary due to the continued variations in the flow rates and temperatures of the seal oil, as well as flow rates and temperatures of the cooling water supplied for controlling the temperatures. Thus, constant attention of an operator is required.
In a second method, individual automatic temperature controllers are provided for each cooler, with the temperature controllers set at the same temperature. The use of two controllers also requires adjustments to ensure that the temperatures are similar, since as the cooling water temperature varies, the desired flow response to a particular temperature deviation will also vary. In addition inherent system deviations make it difficult to maintain a small temperature differential. A major problem and expense involved with the use of separate controls is due to the requirement that the temperature control is finely tuned such that each system accurately responds to the set temperature, with the two controllers having a combined tolerance which ensures the required minimal temperature differential.
Co-pending application Ser. No. 07/646,925 now U.S. Pat. No. 5,147,015 discloses a system for providing seal oils to a gland seal ring which can accurately and economically control the temperatures to reduce or minimize temperature differentials between the oil supplies. The system eliminates the need for constant adjustment as is the case with manual control or separate individual controllers, by utilizing the air-side temperature as a reference for controlling the hydrogen-side oil temperature.
An additional lubrication system is provided to supply oil to the generator bearings, which controls the temperature supplied to the bearings independently of the seal oil temperature. Heretofore, not much attention has been focused on the temperature difference existing between the bearing seal oil and the gland seal oil systems. It has been discovered by the Applicants that it is also beneficial to maintain the seal oil temperatures higher than the bearing oil temperatures. The seal rings are adjacent to the bearings, and the shaft temperature-- and subsequently its diameter--is affected by the changes in bearing oil temperatures. The brass seal ring dimensions change to a greater extent with changes in temperature than the steel generator shaft. Low seal oil temperatures decrease the clearance between the shaft and the ring. This can also help produce "rubs" between the shaft and ring and cause damage thereto.
What is needed then is a arrangement for controlling the temperatures of the gland seal oil system and the bearing lubrication oil system which can maintain a preferred temperature difference therebetween. Preferably such an arrangement is capable of automatic operation to obviate the need for continual manual operation, and minimize the complexity required for the control scheme.